Recent developments in the application of molecular biology to epithelial cancers have led to the identification of specific genetic lesions resulting in either activation or inactivation of key target genes. These genes, called oncogenes, are involved in various aspects of cell growth regulation and as such play major roles in the early carcinogenic processes of "initiation" and "promotion." It is now critical to understand the precise mechanisms by which these genes function so molecular or pharmacologic agents can ultimately be derived to alter or repress their effects. The purpose of this project is to elucidate the biochemical and molecular mechanisms by which oncogenes transform mammalian cells. To this end, we have performed structure/function analysis on members of the myc, jun and fos oncogene families. These studies have revealed various structural aspects of these proteins which are necessary and sufficient for transformation. Our studies of the c-jun oncogene revealed that in addition to the DNA binding and dimerization domains, the N-terminal transactivation domain is required for cellular transformation. In addition, the ability of c-jun to transactivate correlates with its ability to transform cells. Thus, c-jun appears to transform cells by regulating gene expression. Further, detailed mutation analysis of c-jun has demonstrated that phosphorylation of cJun at serines 63/73 results in increased transactivation and ultimately transformation. The phosphorylation of these sites occurs in part through a ras/raf dependent pathway which provides an important biochemical link between these oncogenes. More recent studies are aimed at a more detailed analysis on other c-jun post-translational modications and their biochemical and biologic effects and parallel studies with the c-fos oncogene examining the relationship between phosphorylation and biologic activity. Our studies of the myc oncogene have focused on comparing the transactivating and transforming activities of the c-myc and L-myc genes. By exon shuffking, we have demonstrated that L-myc transactivates and transforms much less ef~ciently than c-myc and this difference is localized to the second exon. More recent work has focused on the precise structural differences between these genes and their role in apotosis.